Direct Heated Solar Collector

ABSTRACT

The presented solar collector panel based on direct solar absorption, offers significantly higher efficiency over the traditional tube and fins designs. This is achieved by containing the heat exchanging fluid under the whole area of the absorber, this reducing its temperature, the main condition for the high efficiency. The heat exchange is further improved by increasing the heat dissipation area. Meandering channel design ensures uniform temperature distribution, which also improves efficiency. In addition to the patent claims, a system with individual panel control is described, taking advantage of different roof slopes and sun movement throughout the day.

1. GENERAL DESCRIPTION

The solar collector described below has been developed as a means to save hot water bills in a private household, without any prior knowledge of the existing designs. Apart from hot water, it can be used in a variety of applications including space heating and horticulture.

It is remarkably simple to manufacture and yet achieves the efficiency close to the theoretical maximum, surpassing existing designs.

1.1. Principal Features

The principal design features of the presented collector system are as follows:

-   -   1. Direct heat transfer from the absorber plate to the heat         exchanging fluid     -   2. Reduction of the overall temperature of the absorber     -   3. Uniform distribution of the absorber temperature     -   4. Increasing dissipation of energy into the heat exchanging         fluid     -   5. Coating of the internal absorber surface to enhance the heat         radiation into the fluid     -   6. Individual panel control, enabling better utilisation of the         solar energy throughout the day     -   7. Reduction of heat loss in pipes, through the improved control         system     -   8. In addition, an additional implementation method is described         suiting less demanding manufacturing facilities

Items 1 to 7 all contribute to the increased collector efficiency, while item 8 refers to the different implementation enabling building the collector in a less technically demanding environment by private persons or hobbyists. The mentioned features are discussed in the body of this document.

The design uses a standard solar glass cover as found in existing flat panel collectors.

FIGS. 1 and 2 illustrate the operating principles of the presented solar panel.

1.2. Efficiency Improvement

The theoretical analysis of the collector is presented in Appendix 1. The efficiency improvement has been calculated with respect to the tube and fins design, however, it is clear that its efficiency is also better than the evacuated tube designs due to the higher effective area of the absorber with the same outside dimensions of the panel.

1.2.1. Direct Heat Transfer

It can be shown using the basic principles of thermodynamics (see Appendix 1) that the direct absorption of solar energy is more efficient than the indirect heat transfer common in the commercially available designs. Intuitively this can be explained by the fact that the collector contains the fluid directly under its practically whole surface, which is not the case in the existing designs. The high volume of fluid lowers the absorber temperature thus increasing the transfer of energy. This is a known phenomenon in physics that the flow of energy (efficiency) increases with the temperature difference. The example is the car engine, which for proper operation needs cooling and loses power when it overheats.

1.2.2. Improved Heat Dissipation

The heat dissipation into the heat exchanging fluid is improved by increasing the effective heat dissipating area of the absorber, through corrugation or heat dissipating ribs and by coating of the internal absorber surface with the radiation enhancing material. It is discussed later in more detail.

1.2.3. Uniform Temperature Distribution

The efficiency is further improved by keeping the panel temperature uniform. This is achieved by horizontal arrangement of the circulating channels, preventing the heated fluid rising to the top as in the thermosiphon solutions. This efficiency improvement is more pronounced with respect to the panels with selective coating of the outside absorber surface, due to high temperature difference between the top and the bottom in the traditional thermosiphon type designs (see Appendix 2).

The temperature sensor located at the collector panel measures temperature of the whole collector, rather than just the local temperature like in the thermosiphon designs. The sensor arrangement is discussed in Sections 2 and 9.1.

1.3. Design of the Circulating Channels

The separation between the absorber part of the panel and the base must be relatively small to minimise the thermal resistance within the heat exchanging fluid. At the same time it must be sufficient to allow unimpeded circulation of the fluid within the panel. Three-dimensional view of the panel base with the die formed channels is shown in FIG. 3.

1.4. Prevention of Heat Loss

The glass pane above the absorber prevents the air heated by the absorber from rising into the outside space, thus limiting heat loss through convection. In addition, the bottom and side walls are insulated with a heat resistant styrofoam.

2. SEQUENTIAL AND COMPLETE HEAT TRANSFER

Thanks to the monitoring sensors and the meandering channel design, when the fluid temperature reaches the appropriate level, all the energy from the collector is transferred to the storage tank in a sequential manner. After the whole of the panel and the downpipe are filled with the cooler fluid, the circulation is stopped by the control system.

This approach ensures that the pump switches on only when the overall panel temperature reaches the required level, rather than, practically, continually working as in the thermosiphon design. This extends the working life of the pump and reduces the consumption of electrical energy.

3. ABSORBER

The absorber in the presented design has a dual function. Its purpose is not only to maximise the absorption of the solar energy but also to maximise heat dissipation into the fluid. This is achieved by corrugation and coating of both the external and internal surfaces. In addition, the absorber should be durable.

3.1. Corrugation

The cross-section A-A in FIG. 1, apart from the fluid circulating channels, shows corrugation in the absorber improving the heat dissipation into the fluid. The purpose of the corrugation is to increase the absorber area being in contact with the fluid thus increasing the heat transfer.

The concave design of the collector cross-section also improves the overall thermal conduction by thinning the layers of the heat exchanging fluid. At the same time, the volume of fluid heated by a comparable absorber area if much higher than in the existing designs, resulting in higher collector efficiency. While the effective solar radiation capture area is defined by the outline of the absorber and does not increase with corrugation, the corrugation will increase the heat dissipation into the fluid.

The absorber corrugation can be created by die forming from a sheet of metal and should agree with the flow of the fluid.

3.2. Heat Dissipating Ribs

The corrugation shown in FIG. 1, cross-section, is only one of the ways to increase the heat dissipation area. This can also be achieved by attaching perpendicular ribs to the absorber as shown in FIG. 5.

3.3. External and Internal Coating

To further increase the heat transfer into the fluid, the inside surface of the absorber can be coated with a heat emission enhancing substance. In its simplest form this can be a durable black paint. Ideally, however, it should be a coating with selective radiation properties in the infrared region, inverting the process of selective absorption of the outside surface.

The outside surface of the absorber can be covered with a known selective material like black chrome or TiNOX or a simple non-selective black paint.

3.4. Absorber Material

The absorber plate needs to be reasonably thin to pose low thermal resistance, but at the same time maintaining reasonable rigidity. 0.5 mm stainless steel will meet these j requirements, offering at the same time excellent durability. Although the rigidity of the absorber is helpful, it is not critical, as this function will be performed by the panel base. A degree of the absorber rigidity will also be provided by corrugation.

4. PANEL BASE

The panel base acts as a supporting frame for the absorber, while at the same time holding the heat exchanging fluid. It can be economically manufactured by die forming from a sheet of metal. In this way, all the panel base features, like channel ridges and side walls, can be produced in one process. The panel base material must be durable and easy to use in the manufacturing process. The same type of material as the absorber plate will facilitate welding, ensure the same thermal expansion coefficient, and reduce the risk of corrosion.

Stainless steel sheets meet the above requirements. Other material could be used providing that they meet the durability and manufacturability criteria.

5. GLASS COVER

The glass cover can be made from standard low iron, solar, toughened glass available from glass manufacturers. It passes approximately 92% of the solar energy and ensures good incidence angles. The glass cover is to be sealed to prevent condensation, which would reduce the effectiveness of the collector.

6. ASSEMBLY

The absorber plate can be attached to the bottom part of the collector by spot welding. The glass is to be mounted on supports attached to the panel frame. The assembly process can be easily automated with modern technology.

7. TUBULAR IMPLEMENTATION

The implementation described so far, although simple and economical, requires investment into the equipment and tooling to achieve the required forms of the panel base and of the absorber.

A simpler form, although slightly less efficient, is presented in FIG. 12. It can be built by a private person using generally available tools and materials.

It consists of contiguous tubes joined together to enable flow of the heat exchanging fluid in a manner analogous to the die formed panel presented earlier in this patent description.

It is based on the same principle of direct heat absorption by the whole panel area, although without improved heat dissipation by profiling and coating of the inside of the absorber. The external surface, however, can be coated with generally available selective coating methods, like electroplating with black chrome.

The solar absorption is performed directly by the tubes. Their tight packing is essential to maximise the absorption. Due to the difficulty to achieve the 180° tube bending, the panel can be built by interweaving copper tubes using available fittings in the manner shown in the FIGS. 12, 13 and 14.

The tubes in the Figures are shown in two different shades of grey for the sake of presenting the idea of interweaving. In real working panel, they all are coated with the same type of absorption enhancing material.

The obvious construction material is the copper tubes, which have very good thermal conductivity, are readily available with a range of fittings and are easy to join by soldering. The tubes should have a reasonably small diameter to ensure fast heating of the fluid, but at the same time enabling proper fluid circulation. The cost is also a factor as the assembly and material will be more expensive in case of the lower diameter tubes.

The horizontal fluid circulation in the tubular implementation, offers the same temperature distribution as the die formed panel. Although the exact volume of the fluid depends on the detailed design and might be lower due to space taken by the tube walls, it is still several times higher than in the available products, achieving higher efficiency.

The efficiency analysis presented in Appendix 1, also applies to the tubular collector.

Looking at the design in FIG. 12, one might be tempted to loop back the fluid at the top left corner so that the collector input and output are at the same point. This has been avoided on purpose, to separate thermally the incoming fluid from the outgoing and ensure proper, sequential heat transfer.

The drawings 12, 13 and 14 present only the essential parts of the tubular design. The complete implementation will include frame, glass cover and insulation in a manner similar to the collector panel presented in the earlier sections of this patent description.

8. INDIVIDUAL COLLECTOR CONTROL

Currently available pumped systems use only a single, common temperature sensor for all the collectors.

Occasionally the orientation and construction of the roof is such that it is difficult to make the collectors face the equator (north or south depending on the hemisphere). The solution is to use separate temperature sensors and remotely controlled valves for the individual collectors and turn the collectors ON or OFF according to their temperature. It is also possible to use common sensor for the individual groups of collectors as shown in FIG. 16.

The schematic diagram of the described system is shown in the FIG. 15. This arrangement will allow, for example, the eastern collector to capture the morning sun, while the western collector would work in the afternoon. The individual collector control will also enable more flexible system design and more efficient utilisation of sunshine on complex roofs. This solution is especially useful for space heating requiring solar collector system with larger area.

It is common during the cooler periods of the year that on a sunny day it is cold inside, while it is pleasantly warm in the sun outside. Individual control of the collector panels will improve the heat transfer from outside into the house throughout the day. A suitably big hot water tank will store the energy to be used when required. This will especially benefit the parts of the house not facing the sun at a particular time of the day.

9. CONTROL SYSTEM

The control system compares the temperature of the individual collectors or the collector groups with the temperature of the hot water tank. When the collector temperature exceeds the temperature of the tank by a set difference, usually 6° C. to 10° C., the control system activates a given collector or a collector group. The present day electronics allows multiple configurations of the control system.

9.1. Complete Heat Transfer Sensor

In a sequential heat transfer, described in Section 2, using a two sensor solution (at the panel and at the storage tank) to activate and to stop the pump, would switch off the pump immediately after the cooler fluid reaches the sensor at the collector output. This would result in the hot fluid in the downpipes being trapped, dissipating the heat without reaching the storage tank. To prevent this from happening, an additional sensor can be fitted at the inlet to the storage tank, ensuring that the pump switches off only after the complete heat transfer into the storage tank.

APPENDIX 1 Analysis of Efficiency

The analysis presented below compares the traditional tube and fins design with the one described in the main body of this document. The tube and fins design is shown in Figure A4.

The Stefan-Boltzmann law states that the power radiated by a body is described by the following formula:

$\begin{matrix} {\frac{E}{t} = {{Ae}\; \sigma \; T^{4}}} & {{Equation}\mspace{14mu} {A1}{.1}} \end{matrix}$

Where:

-   -   T is the body temperature in Kelvins.     -   A is the area of the body.     -   σ is the Stefan-Boltzmann constant equal 5.67 10⁻⁸ (W/(m²K⁴).     -   e is the emissivity determined by the body's surface, with value         between 0 and 1.

Imagine an object as in Figure A1.1, radiating power P_(s). From equation A1.1 we can calculate the object temperature T_(s).

$\begin{matrix} {T_{S} = {\,\sqrt[\frac{1}{4}]{\frac{P_{S}}{e\; \sigma \; A}}}} & {{Equation}\mspace{14mu} {A1}{.2}} \end{matrix}$

Where P_(S) is solar power and T_(S) is the equivalent solar temperature.

Assuming the maximum solar power available on Earth equal 1000 W/m the equivalent solar temperature is obtained as 364K, which is 91.26C. This temperature is reduced depending on the geographical latitude, date and the time of the day as those factors affect the length of the path in the atmosphere traversed by the sun rays and thus affect the absorption.

The Stefan-Boltzmann equation further states that if there are two objects of temperature T₁ and T₂ and all the radiation of the first object is directed towards the second one and vice versa, then the power transferred between the objects will be:

P _(a) =Ae ₁ σT ₁ ⁴ −Ae ₂ σT ₁ ⁴ =Aσ{e ₁ T ₁ ⁴ −e ₂ T ₂ ⁴}  Equation A1.3

Where P_(a) is the absorbed power.

Taking e₁ as 1 for solar radiation, and using power density instead of power, equation A3) can be rewritten as:

P _(d)(x)=dxσ{T _(S) ⁴ −eT _(a) ⁴(x)}  Equation A1.4

Where: T_(S) is the equivalent solar temperature, T_(a)(x) is the temperature distribution of the absorber plate, e is the absorber emissivity, is equal to its absorptivity.

Given the temperature distribution across the absorber plate, one can calculate the power absorbed by the collector, by integrating Equation A1.4 over the entire collector area.

For the collector proposed in this application, the temperature can be assumed to be uniform and equal to the temperature of the fluid.

In the case of the tube and fins collector, the temperature increases with the distance from the tube as shown in Figure A1.2.

Where:

-   -   T(x) is the temperature distribution away from the riser tube.     -   T(b) is the temperature at point b     -   T_(s) is the equivalent solar temperature     -   T₁ is the temperature of the tube     -   b is the half distance between the riser tubes     -   The capital T is used to denote the temperature in Kelvins,         while lower case t is used for temperature in ° C.

The temperature distribution depends on the equivalent solar temperature, distance between the riser tubes and the thickness of the absorber plate. The thicker the plate, the lower is the temperature at the distance b from the tube.

The distribution in Figure A1.2 has been calculated numerically with the following assumptions: t_(s)=90C, t_(t)=30C, b=34 mm, plate thickness=0.2 mm, tube diameter 10 mm. The temperature at point b has been calculated as 78.6C. The tube and fins collector data have been taken from the data sheet of an existing market product.

Dashed line represents uniform temperature distribution for the proposed collector.

Figure A1.3 shows the density of power received by the absorber for both cases, calculated using Equation A1.4.

The area in the Figure A1.3, under the curves plus the middle area corresponding to the tube, represent the received power by the tube and fins collector, while the area under the dashed line corresponds to the described design. Integrating both power densities, results in the total power per square meter equal 245 W and 467 W respectively. This gives efficiency improvement of approximately 90% for the design described in the application.

Note that the above considerations apply also to the tubular implementation, providing that the tubes are arranged contiguously with respect to each other.

The presented analysis is approximate, for example, it does not take into account different heating rates in both collector types and the transfer of heat into the fluid. It is, however, sufficient to illustrate the principle of thermal systems that the efficiency of heat transfer increases with the temperature difference. In this case the difference is between the equivalent solar temperature and the temperature of the absorber.

APPENDIX 2 Comparison of Efficiency Between Panel with a Uniform Temperature Distribution and a Thermosiphon

To start with, consider the power absorbed by the panel of width w and height h, with uniform distribution of temperature.

Based on Appendix 1, the power absorbed by the panel can be described by Equation A2.1.

P _(U) =wheσ(T _(S) ⁴ −T _(U) ⁴)  Equation A2.1

Where:

-   -   P_(U) is the absorbed power for panel with uniform temperature.     -   w and h are defined in Figure A2.1.     -   T_(s) is the equivalent solar temperature as defined in Appendix         1.     -   T_(U) is the panel temperature, in this case, uniform across the         whole panel area.     -   e and σ are defined in Appendix 1, Equation A1.1.

In case of a thermosiphon panel, the temperature distribution can be described as in the Figure A2.2.

Mathematically it is represented by the formula:

$\begin{matrix} {{{T(y)} = {T_{\min} + {\Delta \; T\frac{y}{h}}}}{{{Where}\text{:}\mspace{14mu} \Delta \; T} = {T_{\max} - T_{\min}}}} & {{Equation}\mspace{14mu} {A2}{.2}} \\ {P_{ths} = {w{\int_{0}^{h}{{P_{d}(y)}{y}}}}} & {{Equation}\mspace{14mu} {A2}{.3}} \end{matrix}$

Where: P_(ths) is the total power absorbed by a thermosiphon panel

-   -   P_(d) is the local power density.

Based on Appendix 1, the power absorbed by a thermosiphon panel can be described as:

$\begin{matrix} {{P_{ths} = {{we}\; \sigma {\int_{0}^{h}{\left\{ {T_{S}^{4} - \left( {T_{\min} + {\Delta \; T\frac{y}{h}}} \right)^{4}} \right\} {y}}}}}{Substitute}} & {{Equation}\mspace{14mu} {A2}{.4}} \\ {{{T_{\min} + {\Delta \; T\frac{y}{h}}} = g}{{Thus}\text{:}}} & {{Equation}\mspace{14mu} {A2}{.5}} \\ {{{dy} = \frac{hdg}{\Delta \; T}}{and}} & {{Equation}\mspace{14mu} {A2}{.6}} \\ {{P_{ths} = {{we}\; {\sigma \left( {{hT}_{S}^{4} - {\frac{h}{\Delta \; T}{\int_{T_{\min}}^{T_{\min} + {\Delta \; T}}{g^{4}{g}}}}} \right)}}}{and}} & {{Equation}\mspace{14mu} {A2}{.7}} \\ {P_{ths} = {{hwe}\; \sigma \left\{ {T_{S}^{4} - {\frac{1}{5\Delta \; T}\left( {T_{\max}^{5} - T_{\min}^{5}} \right)}} \right\}}} & {{Equation}\mspace{14mu} {A2}{.8}} \end{matrix}$

Assuming the uniform temperature of a panel as the average temperature of a theimosiphon panel, T_(U) can be expressed as

$\begin{matrix} {T_{U} = \frac{T_{\min} + T_{\max}}{2}} & {{Equation}\mspace{14mu} {A2}{.9}} \end{matrix}$

The efficiency improvement of the panel with uniform temperature distribution over the thermosiphon panel, can be calculated as

$\begin{matrix} {I = \frac{P_{U} - P_{ths}}{P_{ths}}} & {{Equation}\mspace{14mu} {A2}{.10}} \end{matrix}$

Where P_(U) and P_(ths) have the meaning as described earlier.

Using equations A2.1 and A2.8 the efficiency improvement can be represented as:

$\begin{matrix} {I = {\frac{T_{S}^{4} - T_{U}^{4}}{T_{S}^{4} - \frac{T_{\max}^{5} - T_{\min}^{5}}{5\Delta \; T}} - 1}} & {{Equation}\mspace{14mu} {A2}{.11}} \end{matrix}$

The efficiency improvement becomes more visible with the increase in temperature difference between the bottom and top of the thermosiphon panel. For example: assume Ts=361K(88° C.), for T_(min)=283K(10° C.) and T_(max)=351K(88° C.), the efficiency improvement is 5.34%. 

1. Panels described in this application and shown in the FIGS. 1, 3, 4, 5, 6 as well as in the FIGS. 12, 13, 14, maximise the solar absorption through the direct heat transfer from the absorber into the heat exchanging fluid.
 2. The panels as described in the FIGS. 1, 3, 12, 13 and 14, and with regard to claim 1, achieve the complete heat transfer from the collector panel to the storage tank by sequential flow of the heat exchanging fluid.
 3. The solar panels as referred to in claim 1, achieve higher efficiency by lowering the temperature of the absorber through cooling of the whole absorber area with a higher volume of the heat exchanging fluid. This applies also to the tubular implementation as described in Section 7 of this application and in FIGS. 12, 13 and
 14. 4. Increasing the efficiency of the panels, referred to in claims 1, 2 and 3, is, in addition to the earlier claims, achieved through uniform distribution of temperature by avoiding the thermosiphon effect.
 5. Increased dissipation of heat into the heat exchanging fluid is achieved by increasing the heat dissipation area. This can be done, for example, as in the FIGS. 4 and
 5. 6. The efficiency of a solar collector panel is improved by coating the absorber inner surface with the heat radiation enhancing substance.
 7. The utilisation of solar energy throughout the day is optimised by the individual control of collectors as described in Section 9 of this application and shown in FIGS. 15 and
 16. 8. Solar collector panels, as referred to in claims 1, 2, 3 and 4, can be implemented by means of contiguous tubes.
 9. The dense packing of tubes to maximise solar absorption, as referred to in claim 1 and realising sequential flow, as referred to in claim 2, can be achieved by interweaving the tubes in a manner described in Section 7 of this application and shown in FIGS. 12, 13 and
 14. 10. The loss of heat in the pipes, carrying the heat exchanging fluid to the storage tank, is minimised by installing the additional temperature sensor at the inlet to the storage tank as described in the Section 9.1 and in the FIG.
 15. 